Very Recent Advances in Vinylogous Mukaiyama Aldol Reactions and Their Applications to Synthesis

It is a challenging objective in synthetic organic chemistry to create efficient access to biologically active compounds. In particular, one structural element which is frequently incorporated into the framework of complex natural products is a β-hydroxy ketone. In this context, the aldol reaction is the most important transformation to generate this structural element as it not only creates new C-C bonds but also establishes stereogenic centers. In recent years, a large variety of highly selective methodologies of aldol and aldol-type reactions have been put forward. In this regard, the vinylogous Mukaiyama aldol reaction (VMAR) became a pivotal transformation as it allows the synthesis of larger fragments while incorporating 1,5-relationships and generating two new stereocenters and one double bond simultaneously. This review summarizes and updates methodology-oriented and target-oriented research focused on the various aspects of the vinylogous Mukaiyama aldol (VMA) reaction. This manuscript comprehensively condenses the last four years of research, covering the period 2016-2019.

Iridium-Catalyzed Regio- and Enantioselective Allylic Substitution of Trisubstituted Allylic Electrophiles

The first Ir-catalyzed enantioselective allylation of trisubstituted allylic electrophiles has been developed. Through modification of the leaving group of allylic electrophiles, we found that trisubstituted allylic phosphates are suitable electrophiles for asymmetric allylation. The reaction of allylic phosphates with enol silanes derived from dioxinones gave allylated products in good yields with high enantioselectivities.

Total Synthesis of Putative 11-epi-Lyngbouilloside Aglycon

We report here the total synthesis of 11-epi-lyngbouilloside aglycon. Our strategy features a Boeckman-type esterification followed by a RCM to form the 14-membered ring macrolactone and a late-stage side chain introduction via a Wittig olefination. Overall, the final product was obtained in 20 steps and 2% overall yield starting from commercially available 3-methyl-but-3-enol. Most importantly, the strategy employed is versatile enough to eventually allow us to complete the synthesis of the natural product and irrevocably confirm its structure.

Iridium-catalyzed regio- and enantioselective allylic substitution of silyl dienolates derived from dioxinones

Reported herein is the iridium-catalyzed regio- and enantioselective allylic substitution reactions of unstabilized silyl dienolates derived from dioxinones. Asymmetric allylic substitution of a variety of allylic trichloroethyl carbonates with these silyl dienolates gave γ-allylated products selectively in 60-84% yield and 90-98% ee.

Lyngbouilloside and Related Macrolides from Marine Cyanobacteria

Lyngbouilloside and the related macrolides lyngbyaloside lyngbyaloside B and lyngbyaloside C have attracted a lot of attention over the past decade due to their intriguing architecture their natural scarcity and their potential biological activities. This review aims to showcase the various strategies that have been used to access these natural products.

Chemical constituents of the bark of Machilus wangchiana and their biological activities

Eleven new metabolites, butanolides 1-6, lignan derivatives 7-9, sesquiterpene 10, and 3',4'-seco-flavane derivative 11, have been isolated from an ethanol extract of Machilus wangchiana. Twenty known compounds, including ginkgolides A and B (16 and 17), were also isolated. Their structures and absolute configurations were determined by spectroscopic and chemical methods. Compounds 7, 8a, 8b, 9, 11, (+)-guaiacin (12), meso-dihydroguaiaretic acid (13), and hamabiwalactone A (15) showed potent in vitro activities against the release of beta-glucuronidase in rat polymorphonuclear leukocytes (PMNs) induced by platelet-activating factor (PAF), with 42.5-75.6% inhibition at 10(-5) M. Compounds 8, 8a, 8b, 9, and 11 reduced dl-galactosamine (GalN)-induced hepatocyte (WB-F344 cells) damage with 39.4 +/- 6.3% to 53.6 +/- 3.5% inhibition at 10(-4) M. Isomahubannolide-23 (14) was cytotoxic against human stomach cancer (BGC-823) and ovarian cancer (A2780) cell lines, with IC(50) values of 0.13 and 2.66 muM, respectively.

Total synthesis and stereochemical reassignment of (+)-dolastatin 19

[reaction: see text] A revised configurational assignment for the cytotoxic marine macrolide dolastatin 19 is proposed and validated by total synthesis. Key features of the route include an asymmetric vinylogous aldol reaction to install the isolated C13 stereocenter and (E)-trisubstituted alkene, two sequential 1,4-syn boron-mediated aldol reactions, and a Mukaiyama glycosylation to append the l-rhamnose-derived pyranoside.

Hydrogen bond catalyzed enantioselective vinylogous Mukaiyama aldol reaction

[chemical reaction: see text]. The concept of hydrogen bonding catalysis was extended to the vinylogous Mukaiyama aldol reaction, which gives rapid access to polyketide derivatives. The reaction of the silyldienol ether shown and a range of aldehydes catalyzed by TADDOL proceeds regiospecifically to produce the addition products in good yields and enantiomeric excesses.

Ru-catalyzed alkene-alkyne coupling. Total synthesis of amphidinolide P

A coordinatively unsaturated ruthenium complex catalyzed the formation of a carbon-carbon bond between two judiciously chosen alkene and alkyne partners in good yield, and in a chemo- and regioselective fashion, despite the significant degree of unsaturation of the substrates. The resulting 1,4-diene forms the backbone of the cytotoxic marine natural product amphidinolide P. The alkene partner was rapidly assembled from (R)-glycidyl tosylate, which served as a linchpin in a one-flask, sequential three-components coupling process using vinyllithium and a vinyl cyanocuprate. The synthesis of the alkyne partner made use of an unusual anti-selective addition under chelation-control conditions of an allyltin reagent derived from tiglic acid. In addition, a remarkably E-selective E2 process using the azodicarboxylate-triphenylphosphine system is featured. Also featured is the first example of the use of a beta-lactone as a thermodynamic spring to effect macrolactonization. The oxetanone ring was thus used as a productive protecting group that increased the overall efficiency of this total synthesis. This work was also an opportunity to further probe the scope of the ruthenium-catalyzed alkene-alkyne coupling, in particular using enynes, and studies using various functionalized substrates are described.

Catalytic, Enantioselective, Vinylogous Aldol Reactions

In 1935, R. C. Fuson formulated the principle of vinylogy to explain how the influence of a functional group may be felt at a distant point in the molecule when this position is connected by conjugated double-bond linkages to the group. In polar reactions, this concept allows the extension of the electrophilic or nucleophilic character of a functional group through the pi system of a carbon-carbon double bond. This vinylogous extension has been applied to the aldol reaction by employing "extended" dienol ethers derived from gamma-enolizable alpha,beta-unsaturated carbonyl compounds. Since 1994, several methods for the catalytic, enantioselective, vinylogous aldol reaction have appeared, with which varying degrees of regio- (site), enantio-, and diastereoselectivity can be attained. In this Review, the current scope and limitations of this transformation, as well as its application in natural product synthesis, are discussed.

Stereocontrolled total synthesis of (-)-callipeltoside A

[structure: see text] A highly stereocontrolled total synthesis of the cytotoxic macrolide (-)-callipeltoside A has been achieved in 23 steps (4.8% overall). Notable features include a novel asymmetric vinylogous aldol reaction to install the C13 stereocenter and (E)-trisubstituted alkene, an anti-selective aldol addition, a Sonogashira coupling, and, last, a Schmidt-type glycosylation to attach the sugar unit.

Versatile Asymmetric Synthesis of the Kavalactones: First Synthesis of (+)-Kavain

[reaction: see text] Three asymmetric pathways to the kavalactones have been developed. The first method is chiral auxiliary-based and utilizes aldol reactions of N-acetyl thiazolidinethiones followed by a malonate displacement/decarboxylation reaction. The second approach uses the asymmetric catalytic Mukaiyama additions of dienolate nucleophile equivalents developed by Carreira and Sato. Finally, tin-substituted intermediates, prepared by either of these routes, can serve as advanced general precursors of kavalactone derivatives via Pd(0)-catalyzed Stille couplings with aryl halides.

α-Heterosubstituted Aldehydes in Organic Synthesis. Enantioselective Approaches to New Analogues of Mevinic Acids

Various aldol approaches towards the asymmetric synthesis of the lactone moiety of HMG-CoA-reductase inhibitors are described. Auxiliary controlled as well as catalytic aldol reactions resulted only in modest to low selectivities, whereas 1,2-additions to readily available highly enantiomerically enriched α-heterosubstituted aldehydes yielded δ-hydroxy-β-ketoesters with a high degree of diastereocontrol and in good chemical yields. The novel mevinic acid analogues could then be obtained bysyn-reduction of the addition products.

Regioselective Oxygenations of S-Trans Dienes, Silyl Dienol Ethers (SDEs), by Triphenyl Phosphite Ozonide (TPPO) and Its Mechanistic Study

The direct oxygenation of s-trans dienes, silyl dienol ethers (SDEs) 2, by triphenyl phosphite ozonide (TPPO) has been examined in detail. The regioselective oxygenation was found to give hydroperoxide 3, alcohol 4, ketone 5, dimer 6, and peroxy phosphate 7 with concomitant formation of triphenyl phosphate 8 and diphenyl trimethylsilyl phosphate 10. The formation of peroxy phosphate 7 was found for the first time in TPPO oxygenation reactions. The low temperature (31)P and (1)H NMR spectroscopic analyses proved the direct reaction of SDEs with TPPO without generation of singlet oxygen. The formation of the oxygenated products 3-7 is reasonably explained by the intervention of the zwitterion ZI, which can be formed by the nucleophilic attack of SDE to the central oxygen of the ozonide. The regioselective attack of SDE to the central oxygen of the ozonide was supported by the quantum chemical calculation (B3LYP/6-31G).

Postulated biogenesis of WS9885B and progress toward an enantioselective synthesis

[formula: see text] WS9885B promotes the assembly of microtubules in vitro and displays cytotoxicity as potent as paclitaxel against several cancer cell lines. In this Letter, we propose a biogenesis for this architecturally complex bacterial metabolite from a much simpler, polyunsaturated precursor. We also present significant progress toward a convergent, enantioselective synthesis of WS9885B. Our work features a chemoselective palladium-catalyzed cross-coupling of two advanced building blocks and an uncommon Claisen-like cyclization.

Catalytic asymmetric allylation reactions using BITIP catalysis and 2-substituted allylstannanes as surrogates for beta-keto ester dianions

[formula: see text] Catalytic asymmetric allylation (CAA) reactions using the indicated allylstannane and the BITIP catalysts previously described by us give high yields and enantioselectivities in additions to aldehydes. The products are convertible to beta-keto esters by oxidative cleavage of the olefin. These reactions thus provide a useful catalytic enantioselective method for chain extension with introduction of a versatile four-carbon unit.